Fix stack overflow in edge function

Address "Maximum call stack size exceeded" error by chunking base64 image conversion and adding image size validation.

supabase/functions/generate-alt-text/index.ts

···
     }
 
     const imageBuffer = await imageResponse.arrayBuffer();
-    const base64Image = btoa(String.fromCharCode(...new Uint8Array(imageBuffer)));
+    
+    // Check image size (max 10MB)
+    if (imageBuffer.byteLength > 10 * 1024 * 1024) {
+      return new Response(
+        JSON.stringify({ error: 'Image too large. Maximum size is 10MB.' }),
+        { status: 400, headers: { ...corsHeaders, 'Content-Type': 'application/json' } }
+      );
+    }
+
+    // Convert to base64 in chunks to avoid stack overflow
+    const uint8Array = new Uint8Array(imageBuffer);
+    let binaryString = '';
+    const chunkSize = 8192;
+    for (let i = 0; i < uint8Array.length; i += chunkSize) {
+      const chunk = uint8Array.slice(i, i + chunkSize);
+      binaryString += String.fromCharCode(...chunk);
+    }
+    const base64Image = btoa(binaryString);
     const contentType = imageResponse.headers.get('content-type') || 'image/jpeg';
 
     console.log('Calling Claude API...');